RAPID COMMUNICATION Granulocyte Colony-Stimulating Factor Rapidly Activates a Distinct

Binding of granulocyte colony-stimulating factor (G-CSF) to normal myeloid cells activates the protein tyrosine kinases Lyn and Syk and results in the immediate early upregulation of G-CSF receptor (R) mRNA. In our studies of the signaling pathways activated by G-CSF that are coupled to prolifera-tion and differentiation of myeloid cells, we examined whether G-CSF activated a latent transcription factor be-longing to the STAT protein family. Electrophoretic mobility shift assays (EMSAs) of nuclear extracts from G-CSF-stimu-lated human myeloid cells showed the rapid activation of a DNA-binding protein that bound to the high-affinity serum-inducible element (hSIE) and migrated with mobility similar t o serum inducible factor (SIFI-A (Stat3 homodimer). The G-CSF-stimulated SIF-A complex (G-SIF-A) did not bind to duplex oligonucleotides used to purify and characteriz